1. Field of the Invention
The present invention relates to a magnetic recording medium exemplified by a floppy disk, a video tape, and other rigid disk-like mediums. More particularly, it relates to a magnetic recording medium of a ferromagnetic metal thin-film type, having wear resistance, travel stability, and environmental resistance.
2. Related Background Art
In recent years, there is an increasing demand for magnetic recording mediums capable of high-density recording, as information storage requires greater capacity in the field of information processing techniques and as picture or image quality is improved. Under such circumstances, research and development are energetically made so that such demand can be met.
In such research and development, metal thin film magnetic recording mediums comprising a ferromagnetic metal thin film of Co-Ni, Co-Cr or the like as a magnetic recording layer, formed by sputtering or vapor deposition are considered hopeful as mediums which are suitable for high-density recording, compared with coat-type magnetic recording mediums which are commonly used at present.
In the metal thin-film magnetic recording mediums, as distinct from the coat-type magnetic recording medium, the whole thin film contributes to the magnetic recording and also has a high flatness, so that it becomes possible to make spacing loss very small.
If it is possible to utilize the advantages of such metal thin-film magnetic recording mediums, a magnetic recording medium would be obtainable that has both output and S/N ratio greatly surpassing those of the coat-type magnetic recording mediums.
In recording systems, employment of a system made to have narrower tracks and shorter wavelengths enables high-density recording with a density at least several times the density that can be attained by the coat-type magnetic recording mediums.
In the present state of things, however, various problems still remain to be solved for putting the metal thin-film magnetic recording mediums into practical use.
For example, in flexible mediums such as video tapes, video floppy disks, data-recording tapes and floppy disks, the magnetic recording medium travels while coming into partial contact with a magnetic head, and recording or reproduction is thus carried out. Then, the probability of the contact between the magnetic head and medium tends to increase as the spacing loss is made smaller in order to achieve the high-density recording. In rigid magnetic recording mediums comprising a substrate made of nonmagnetic metal or the like, it is common for the medium and magnetic head to come into contact with each other when the medium is standing.
However, a magnetic recording layer comprising the metal thin film is subject to damage as a result of contact with the magnetic head. Once the layer has been damaged, the travel performance becomes poor, resulting in a lowering of output, or sometimes resulting in travel failure. This has been the greatest problem that has hindered the metal thin-film magnetic recording mediums from being put into practical use.
Another problem involved in putting the metal thin-film magnetic recording mediums into practical use is that some type of metals used result in corrosion of the magnetic recording layer of the metal thin film when it is brought into contact with the air for a long period of time. For example, a Co-Ni metal thin film may corrode in a short time in an environment of high temperature and high humidity or the environment wherein salt is in the air.
The coat-type magnetic recording mediums conventionally used in video tapes, video floppy disks, data-recording floppy disks, etc. are formed by mixing magnetic powder in a binder and coating the resulting mixture on a base film. These mediums originally have a finely roughened surfaces such that they have a small frictional resistance. Materials with excellent wear resistance or lubricity are also added in the binder, whereby the problem concerning the sliding to the magnetic head has been settled, thus enhancing overall reliability.
On the other hand, in the metal thin-film magnetic recording mediums, it has been attempted to form a protective layer on the surface of the metal magnetic layer by the methods as described below, in order that the protective layer can impart (1) wear resistance, (2) lubricity and (3) environmental resistance to the magnetic recording medium.
(1) To impart wear resistance
An inorganic protective layer comprising a hard material is formed on the metal thin-film magnetic recording layer.
For example, a thin film comprising SiO, SiO.sub.2, SiN, Al.sub.2 O.sub.3, TiO.sub.2, or diamond-like carbon is formed by vacuum deposition, sputtering, plasma CVD or the like.
(2) To impart lubricity
(i) a protective layer comprising lubricating material(s) is formed on the metal thin-film magnetic recording layer or on the above inorganic protective layer.
For example, a thin film comprising an inorganic material such as MoS.sub.2, WS.sub.2, diamond-like carbon, or amorphous carbon is formed by vacuum deposition, sputtering, plasma CVD or the like. Alternatively, a layer comprising an organic material such as a fluorine resin, a silicone oil surface active agent, a saturated fatty acid or an ester oligomer is formed by the coating-solution coating, vacuum deposition, sputtering or the like.
(ii) fine irregularities are formed on the surface of the magnetic recording medium so that the true contact points may be decreased to lower friction coefficient.
(3) To impart environmental resistance
A corrosion-resistant protective layer is formed.
For example, a layer comprising an elemental single substance such as Al, Cr, Ti, V or Si, a layer comprising an oxide, nitride, carbide, boride or the like of the above elements, a composite layer comprised of these layers, or a layer comprising a polymeric material such as polyethylene, polyimide or nylon is formed by vacuum deposition, sputtering, plasma CVD, coating-solution coating or the like.
The standards of durability for floppy disks by which reliability is measured include checking to ensure that data-recording floppy disks exhibit a continuous-travel durability of not less than 3,000,000 passes at ordinary temperature and not less than 1,000,000 passes at high (about 50.degree. C.) and low temperature (about 10.degree. C.); and that video floppy disks exhibit a continuous-travel durability of, not less than 48 hours (about 10,000,000 passes) at ordinary temperature, high temperature and high humidity (40.degree. C., 85% RH), or low temperature (-5.degree. C.).
The metal thin-film magnetic recording mediums, however, may cause scratches or deposits on the surface of the medium or the surface of the magnetic head after their travel of several ten thousand to several hundred thousand passes at most, resulting in a great lowering of reproduction output. Thus, they can not be said at all to have reached a practical level. The conventional rigid metal thin-film magnetic recording mediums also cause scratches or deposits (aggregation of fine powder) on the surface of the medium or the surface of the magnetic head as a result of CSS (contact start stop) repeated several times to several thousand times. Thus, the reproduction output is lowered to a great extent.
This is because conventional protective layers lack sufficient hardness, so that the protective layer itself is damaged as a result of its sliding to the magnetic head, and the damage of the protective layer extends to the magnetic recording layer. Fine powder produced at this time adheres to the magnetic head to bring about clogging of the head, causing an extreme lowering of reproduction output (for example, a dropout) in some instances, or causing damage such as scratches on the surface of the medium or head in a worst instance. The durability of the medium and head is thus impaired.
A proposal has been made on a magnetic recording medium comprising a protective layer in which an oxide of silicon is used (as disclosed, for example, in Japanese Patent Application Laid-Open No. 61-115229, No. 61-178730 and No. 62-229526). This, however, is still not satisfactory.
In the data-recording floppy disks, there is an example in which a Co.sub.3 O.sub.4 film, serving also as a solid lubricanting film, is used as the protective layer of a Co-Cr magnetic layer, and which shows a still durability of not less than 10,000,000 passes [Samoto et al, SHINGAKU GIHO 87-15 (1987)]. Experiments made by the present inventors, however, have confirmed that the thin film layer comprising Co.sub.3 O.sub.4 can give no satisfactory long-term storage durability and environmental resistance. The conventional metal thin-film magnetic recording mediums cause corrosion on the metal thin-film magnetic recording layer in about 0.1 to 10 hours at best when an environmental durability test is carried out (target specifications: not less than 500 hours under conditions of 85.degree. C., 85% RH). Thus, they have reached no practical level of durability under actual conditions.